A racing Brain teaser

[Guest Dry Reach]

It's simple and nothing to do with most of the theory and lack of information (not given) above (including mine!)

 

 

How many people here have tried to sail upwind in 0 knots of breeze with 5 knts of tide behind you and managed to actually sail let alone have good steerage?

 

5 knots of tide behind you does not equate to 5 knots of sailable breeze

 

thus if your boat is at the mercy of the river flow then the less stuff sticking in the air the faster you will go down stream!

 

so the log wins! in 2 hours (if the flow is constant across the river and not just in the deeper middle)

[Guest]

 

5 knots of tide behind you does not equate to 5 knots of sailable breeze

 

 

I agree,

However, it's also true that when sailing on the wind across a tide running left to right eg the sails have more power on stbd than port. So i guess that must also apply in a calm. Unless, of course the lack of lift from the foils negates that advantage. Dunno. Also what bene says about the friction between the water and air is a huge factor. I love this sh*t, pity I'm too dumb to work it out.

 

Here's another, we just had the solstice so I shot the sunrise and it rose around 30 degrees north of east. Why not 23.4 as in the tropic latitude? Took forever to work out why. Bit nerdy i know but the whole maths thing is fun.

[Marshy 30]

This is like the old question of what weighs more, a ton of feathers, or a ton of lead???

Whether you tack across the river, the water is still moving at 5knots downstream so you still have 5knots vmg whatever way yout facing... You could face the wrong way and still drift...

[Guest]

Like the call I had from an engineer last week wanting to know what a kilo of 21.5mm stud link chain weighs. He was quite happy when I answered 1000 grams. We still can't work that one out, he did appear to be quite serious

[wheels 543]

Density of air = 1.2kg/m^3

Density of sea = 1020kg/m^2 (at a guess)

Hey Clipper, how do you weigh a square metre of water??

 

I am rather glad we aren't all trying to send a Rocket to the moon. Or maybe that is actually easier

[MrWolf 0]

I think MrWolf came through with the answer they will have been looking for.

 

Possibly not the most tidily-constructed question but perhaps the red herrings left lying around are all part of the fun.

 

Broke Mariner, I am guessing seven groundings in total with one RIB having been responsible for three of them. Am I close?

 

Ooh Thank you !

 

I'm going no matter what class of boat, be it opti or BMW Oracle tri its all gonna make it in the same time no matter which way it faces.

I'm saying that because much of the air is moving downstream with and around the objects in the water.

 

the big non issue the question alludes to is the so called "slippage" through the water, of which the "lee- bow effect" is one example.

No matter which way the boat faces in the body of water, it is the body of water that is moving. So facing straight into the tide or at 45 degrees to the tide or at 90 degrees to the tide makes no difference.

[Guest Dry Reach]

Hold your horses!

 

i have an issue!

 

was passing my local, quite wide river" this morning and hopped out and did the following test!

 

the river is a deep, slow, and constant river that was flat and there was no wind.

 

The test...

 

at the same time i placed into the water a large leaf and a heavy log and followed them for 5 minutes!

 

and gues what?...

 

 

The lighter and less submerged leaf "kicked arse"

 

I tried it 3 more times and it won every time.

 

On closer observation it appeared The log took longer to gain momentum due to its size to wieght and never got back that lost initial momentum. the leaf reacted immediately and maintained its speed!

 

I would say that if the river had fast and slow areas the logs momentum would carry it through the slow areas and catch up with the slowing leaf! but on this wide river that did not happen.

 

so the heavy objects loss of initial momentum killed its chances!

 

the lighter boat wins? (if its a drifting race!)

[Theseus 1]

I know what the reverse situation is like.

 

I was crewing in a Stewart Island race and we started with a 5 knot tail wind with kites up.

We entered the the channel at Bluff where a 5 Knot ebb tide was running.

The kites colapsed, the boats all lost all speed through the water, no steerage and the boats all drifted at the same speed sideways down the channel.

Horrible steel tower channel markers then came at us at 5 knots!

Out board up in well with plug in - result -Panic!

We were lucky.

I understand Durville at a similar problem in French pass when he hit a rock.

[Guest Dry Reach]

think about this...

 

 

your drifting downstream at 5 knots (river flow speed) with a 5 knot breeze blowing in the same direction as the 5 knot river flow!

 

which way will your Windex point?

[Guest]

As per theseus, it would do circles.

 

Great that this question has inspired practical experiments but I think we can assume a rolling start in a yacht race.

[B00B00 310]

Come on Wheels whats the answer!!!!

 

I based my answer on a boat with a massive code zero!

[Guest]

I reckon Mr Wolf's got it. Your code zero will be limp as a .................. limp thing.

[Guest Dry Reach]

As per theseus, it would do circles.

 

.

 

 

Well acording to my theory it would point to the apparent wind (zero) and thus sit at it's last position prior to reaching 5 knots speed!

 

think about that

[B00B00 310]

Well anyone who has raced in a lot of tide before knows the effect of being dragged thru the air.

OK so there will be a small amount of air moving with the water, I don’t know exactly how much but it could be that a bit of air 2m-5m high will be moving and staggered so that at 1m its doing 5km and 5m its doing 0.5kms, above that the air will be calm as the question suggests, even a 30fter has sails up at 10+m. Now you drag a boat with light airs sails set, thru that air the sails will set-right? Even if its only just slightly that will produce a small amount of power, enough to push the boat forward at 1kt(?) 50 degrees to the TWA (which is right on the nose). Then any boat speed you are adding is also adding apparent wind. If you are moving thru the water then you are gaining on the guy who is not moving thru the water.

Im not saying you are going to take off to warp speed but I am sure you would be able to get the boat moving, even if its just very slighty.

 

We did exactly the same thing on Maximus in the Solent.

 

When I come over the bridge in the mornings you can see the wind on the water caused by the tide flow, when there is ZERO wind in the rest of the harbor it appears there is wind in the high flow areas like near the ODM.

[Guest Dry Reach]

Here is a Brain Teaser. It is from an engineering Website and it is possible a couple of others here also view this website. The actual answer is going to be posted on the 6th, so even those that have seen this before, can still offer there opinions. Although they most likely have been privy to many of the replies.

So here it is.

 

You are in a sailing race on a very wide river. On land there is no wind; it is a dead calm. The race is 10 Km downstream, and the river is running 5 Km/ hr. Your opponents decide that to make the best time, they will streamline their boats to the wind and float downstream as fast as they can. You decide on a tacking strategy, sailing back and forth across the river's width. Who wins the race? What is the winning time?

 

 

Hold on everybody!!!

 

 

Tacking is certainly faster, with a speed of about 3 km/h through the water being feasible on a fast boat in this breeze. If the boat is well sailed, .7 of this will be VMG, so the speed over the ground will be about 2 km/h + 5 km/h or about 7 km/h. Time would be about 1.4 hours.

 

The drifters would be pushed back a bit by aero drag. So maybe 2.1 hours would be required.

 

[Guest Dry Reach]

problem Houston...

 

 

How deep is the river? and whats the draft of said boats?

 

the may not even get past the start!

 

DNF!

[Guest Dry Reach]

How about this...

 

For all you boffins!

 

Boat one is adrift, but skillfully piloted straight down the middle. It runs at slightly slower that current speed due to drag. It's travel time will be roughly 2 hours plus the coefficient of drag disadvantage. It presents a small cross-section to the current. It never actually reaches 5kph because of mass and drag.

 

Boat two elects tacking, which you understand, and has been explained in depth numerous times above. It gives some 1.x speed advantage. . ANY tacking advantage results in a vector addition of speed down-river. Regardless of how much extra distance they travel. Zig-zagging down-river picking up speed with every change in vector cuts a lot of travel time. BTW - the initial push #2 got was sufficient to begin tack.

 

Both craft start with a vector magnitude of zero. Both are released from restraint at the same time. Craft one will slowly start to pick up speed because it has mass. F=mA. Both F and A are vector numbers. Craft two immediately turns their craft 90 degrees to the current and waves good bye to the other one as they are swept away. Their hull cross-section is much larger, and generates far more Force. If you plot their speed vs distance, you will see that number two immediately has an advantage. The mass also is the same, of course but F is far larger due to the cross-section of the hull, the energy imparted by the same drag the other boat is experiencing, and the apparent wind transferring energy while the sails are in tack configuration.

No, I do NOT subscribe to the theory that the water drag on the other side of craft two will equal the force generated. Try this. Hold a board parallel to any large water flow. Make sure you select a stream that is reasonably fast. Now turn it 90 degrees and see if you can hold on to it. You can even try it with airfoil shapes like boats. They always seek the minimum energy configuration, which is parallel to the current.

 

I stayed away from all the math involved except for Newton's principle. Since this involves vectors, you can use linear algebra but the equations are too simple to bother. I understand the basics of tacking enough to realize that the apparent wind is enough gather energy from.

.

[Guest]

To those who are worried about hydrodynamic drag i.e. the boat going slower than the river coz of drag-this is zero, assume we are past initial boundary conditions (they have already started the race at the max speed they will achieve) the "non sailing" vessel is moving at the same speed as the water therefore no drag.

 

The sailing vessel is "taking" energy from the interface of two fluids moving at different velocities- this is actually a pretty good definition of sailing. (yes the air is a fluid - a very thin one).

[Guest Brucey]

To those who are worried about hydrodynamic drag i.e. the boat going slower than the river coz of drag-this is zero, assume we are past initial boundary conditions (they have already started the race at the max speed they will achieve) the "non sailing" vessel is moving at the same speed as the water therefore no drag.

 

The sailing vessel is "taking" energy from the interface of two fluids moving at different velocities- this is actually a pretty good definition of sailing. (yes the air is a fluid - a very thin one).

 

 

Wrong air is a gas and compressable. Fluids are incompressable. It does have a moisture content but on most occasions the temperature is above the dew point and as such it is suspended in the gas.

 

DR sut up mate you're dribbling.

 

Astounded at the confusion this has caused, but now unsuprised with some of the sh*t you see on the track.

[Guest Rocket]

I detect the curious breed of people that think you should put your boat side on to a current to get more effect (known collectively as the lee-boweres) - I encourage all of them to think about sailing on an escalator.....

 

Just draw all the vectors on a piece of paper and you will see that the "sailor" beats the "drifter" so long as the sailor can generate a positive vmg... And tacking badly won't change the outcome it will just reduce the difference (and could be assumed to be swallowed by using an average vmg).